Wideband multifrequency acoustic transducer

ABSTRACT

The invention relates to multifrequency acoustic transducers exhibiting a wide band around each resonant frequency. It consists in inserting between a λ/2 active emitter plate (201) and the soft reflector (203) which supports it a rear plate (202) resonating in λ/4 mode and in placing on this active plate two marcher plates (204, 205) whose impedances are designed so as to best match the two frequencies obtained by inserting this rear plate. Thicknesses of these marcher plates are optimized with the aid of a model of for example Mason type starting from a value close to λ/4 for the frequency to be matched. It makes is possible to construct sonar transducers which operate equally well in detection mode and in classification mode.

The present invention relates to acoustic transducers capable ofoperating on several emission frequencies and/or of receiving with widepassbands around these frequencies. It makes it possible in underwaterimaging to obtain long range for low frequency, but with low resolution,and high resolution for high frequency, but with short range.Low-frequency operation is then used first to pinpoint the objects whichit is desired to identify. The boat carrying the sonar equipped withthis type of transducer subsequently approaches the object thusdetected, and when sufficiently near, the high frequency is used makingit possible to obtain an accurate image of this object.

It is known from French Patent Application Number 8707814, filed by theapplicant on 4 Jun., 1987 and granted on 9 Dec. 1988 under the number2616240, to fabricate a multifrequency acoustic transducer essentiallyintended to be used in medical uses, by inserting between the activepiezoelectric plate and the reflector of an ordinary probe, a half-waveplate with the natural resonant frequency of this plate. The probe canthus be used at two distinct frequencies, one being substantially equalto half the other. However, this system, although it is well suited tomedical imaging, in particular so as to use one frequency in imagingmode and the other frequency to view blood flows, exhibits a number ofdrawbacks in underwater imaging. In particular, the bandwidth around oneof the two resonant frequencies is relatively small. This is not veryimportant in respect of the frequency used to view blood flows. Inunderwater imaging, by contrast, the processing operations used make itnecessary to have a large bandwidth for both frequency ranges.

To alleviate these drawbacks, the invention proposes a widebandmultifrequency acoustic transducer, of the type comprising apiezoelectric emitter plate of impedance Z and resonating in λ/2 mode ata fundamental frequency F0, a rear plate of impedance Z3 and a supportforming a reflector of the type with substantially zero impedance,characterized in that the rear plate resonates in λ/4 mode at thefrequency F0 so as to make it possible to obtain two resonantfrequencies FA and FB of the assembled transducer, and in that thistransducer furthermore comprises two front matcher plates whoseimpedances Z1 and Z2 are given by the formulae

    Z1≅Z0.sup.3/5 ×Z.sup.2/5

    Z2≅Z0.sup.2/5 ×Z.sup.3/5

and whose thicknesses enable them to resonate at frequenciessubstantially equal to λ/4 for respectively each of the frequencies FAand FB and to be substantially transparent for respectively each of theother frequencies; these thicknesses being optimized with the aid of aMason type model.

According to another characteristic, the rear plate is formed from thesame material as the active plate.

According to another characteristic, the material constituting theactive layer and the rear plate is a ceramic of the PZT type for which Zis substantially equal to 21×10⁶ acoustic ohms, the matcher plates haverespective impedances Z1=3.9×10⁶ acoustic ohms and Z2=6×10⁶ acousticohms, and the thicknesses of these plates are respectively equal as afunction of the frequency which they are required to match to e1=λ/2.16and e2=λ/5.04 at the 1st frequency, and to e1=λ/3.77 and e2=λ/8.81 atthe 2nd frequency.

According to another characteristic, the active plate has a thicknesssuch that it resonates in λ/2 mode at a frequency of 250 kHz and in thatthe two frequencies of emission for which the transducer is matched aresubstantially equal to 350 kHz and 150 kHz.

Other features and advantages of the invention will emerge clearly inthe following description presented by way of non-limiting example withregard to the appended figures which represent:

FIG. 1, a sectional view of the structure of an antenna according to theinvention;

FIG. 2, an exploded perspective view of the various layers, constitutingthis antenna; and

FIG. 3, a perspective view of such a transducer after slicing to obtaincolumns necessary in the case of an application to a sonar.

Represented in FIG. 1 is a section taken through the thickness of atransducer according to the invention.

The active element of the transducer is composed of a piezoelectricceramic plate 201 which resonates in λ/2 mode at a "natural" frequencyF0 when it is isolated. This plate is fixed on a support 203 by way of arear plate 202 which itself resonates in λ/4 mode at F0. The support 203itself constitutes a reflector of the substantially zero impedance type,known in particular by the English term lightweight "backing", or softreflector. To obtain such a substantially zero impedance with a materialstrong enough to bear the transducer, a low-density cellular material isused according to the known art.

Adding the resonating rear plate 202 to the piezoelectric ceramic plate201 makes it possible to obtain two resonant frequencies FA and FB forthe unit as a whole, such that FA lies between 1.5 FB and 3 FB.Furthermore (FA+FB)/2=F0.

So as to improve the behaviour of the transducer, in particular itsmatching with respect to the medium, generally water, in which it isrequired to emit, as well as the obtaining of sufficient bandwidthsaround the two resonant frequencies FA and FB defined above, two frontmarcher plates 204 and 205, each of quarter-wave type at the twofrequencies FA and FB respectively, are overlaid on the front emitterface of the plate 201.

Denoting by Z the impedance of the piezoelectric ceramic, by Z0 theimpedance of the exterior medium into which the acoustic waves areemitted, and by Z3 the impedance of the rear plate 202, it may be shownthat an apt choice of the impedance of the rear plate, Z and Z0 being inprinciple determined by materials used, makes it possible to choose theratio of frequencies FA/FB. Thus, to cover an FA/FB span of from 1.5 to3, it is appropriate to choose Z3 between Z/6.2 and Z×4.6.

In the prior art it was known how to match just a single of the twofrequencies by using a single front matcher plate, except in certainparticular numerical cases, for example when FA/FB=3.

To match both frequencies, the invention therefore proposes to use twofront marcher plates 204 and 205, making each plate particular to onefrequency in such a way that one of the plates matches the device inrespect of one of the frequencies and the other plate in respect of theother frequency. In fact, given that these plates are overlaid, theirbehaviours interfere with one another, essentially insofar as the platesare not completely transparent to the frequencies in respect of whichthey are not matched.

It is therefore desired simultaneously to meet several criteria:

that each plate taken separately should effect impedance matching at thefrequency assigned to it;

that the transmission of acoustic energy emitted by the piezoelectricceramic 201 should be optimized towards the front medium.

Research by the inventors has culminated in determining the impedancesof the two plates according to the following two formulae:

    Z1≅Z0.sup.3/5 ×Z.sup.2/5

    Z2≅Z0.sup.2/5 ×Z.sup.3/5

Furthermore, the invention proposes that the thicknesses of the twofront plates be close to a quarter of the wavelength of the frequenciesFA and FB, and that their exact values be obtained from the use of awell-known model based on the equivalent diagrams published by W. P.MASON in Physical Acoustics Principles and Methods 1964--Academy Press.

By way of example embodiment, use was made of a plate 202 made ofpiezoelectric ceramic of the PZT type exhibiting an impedancesubstantially equal to 21×10⁶ acoustic ohms. The thickness of the plateis chosen so that it resonates in λ/2 mode at a frequency F0=250 kHz.

The rear plate is designed to resonate in λ/4 mode at this samefrequency, and the invention proposes by way of improvement to fabricatethis plate from the same ceramic, of the PZT type, as that used for theactive piezoelectric plate 201. This makes it possible to a large extentto simplify the fabrication of the transducer.

Under these conditions, values substantially equal to 350 kHz and to 150kHz respectively will be obtained for the two frequencies FA and FB. Itis clear that FO is substantially equal to (FA+FB)/2 and thatfurthermore FA/FB is substantially equal to 2.33.

The plates 204 and 205 are made, according to the known art, frommaterials whose composition makes it possible to obtain the desiredacoustic impedances. These impedances will be chosen, in accordance withthe formulae cited earlier, to have values Z1=3.9×10⁶ acoustic ohms andZ2=6×10⁶ acoustic ohms.

The use of the Mason type model to define the thicknesses of these twoplates gives results, expressed in wavelength, equal to:

For FA=350 kHz, e1=λ/2.16 and e2=λ/3.77

For FB=150 kHz, e1=λ/5.04 and e2=λ/8.81

It is therefore observed that in effect for each of the frequencieschosen, the corresponding matcher plate has a thickness substantiallyequal to λ/4, this procuring the desired matching, and that at the otherfrequency, the thickness of the plate is close to λ/2 for one, and lessthan λ/8 for the other, thus rendering them substantially transparent tothe acoustic waves for the frequencies which they are required not todisturb.

The variations with respect to λ/4 and to λ/2 originate precisely fromthe interaction between the various layers, the effect of which ismodelled by the Mason type model.

Measurements performed on a transducer constructed according to thesecharacteristics have shown that the bandwidths obtained were greaterthan 20% for FA and greater than 50% for FB, this being entirelysatisfactory.

In order to make a transducer using this structure, a succession ofplates of the chosen materials with the thicknesses thus determined arestacked, as represented in FIG. 2, furthermore interposing electrodes211 and 221 formed from a slender conducting metallic layer which doesnot disturb the acoustic operation of the unit as a whole, between theceramic 201 and the layer 204 on the one hand, and between this ceramicand the layer 202 on the other hand. These electrodes 211 and 221 jutout from the sandwich in such a way as to be accessible so that they canbe connected to the leads delivering the signal intended to excite theceramic 201. These various plates are glued together, and the sandwichthus obtained is subsequently sliced into columns as represented in FIG.3, so as to obtain the structure of the transducer necessary to obtaincorrect emission of the acoustic waves through the front face, accordingto techniques well known in sonar.

We claim:
 1. Wideband multifrequency acoustic transducer, of the typecomprising a piezoelectric emitter plate (201) of impedance Z andresonating in λ/2 mode at a fundamental frequency F0, a rear plate (202)of impedance Z3 and a support (203) forming a reflector of the type withsubstantially zero impedance, characterized in that the rear plate (202)resonates in λ/4 mode at the frequency F0 so as to make it possible toobtain two resonant frequencies FA and FB of the assembled transducer,and in that this transducer furthermore comprises two front marcherplates (204, 205) whose impedances Z1 and Z2 are given by the formulae

    Z1≅Z0.sup.3/5 ×Z.sup.2/5

    Z2≅Z0.sup.2/5 ×Z.sup.3/5

and whose thicknesses enable them to resonate at frequenciessubstantially equal to λ/4 for respectively each of the frequencies FAand FB and to be substantially transparent for respectively each of theother frequencies; these thicknesses being optimized with the aid of aMason type model.
 2. Transducer according to claim 1, characterized inthat the rear plate (202) is formed from the same material as the activeplate (201).
 3. Transducer according to claim 2, characterized in thatthe material constituting the active layer (201) and the rear plate(202) is a ceramic of the PZT type for which Z is substantially equal to21×10⁶ acoustic ohms, in that the marcher plates (204, 205) haverespective impedances Z1=3.9×10⁶ acoustic ohms and Z2=6×10⁶ acousticohms, and in that the thicknesses of these plates are respectively equalas a function of the wave frequency which they are required to match toe1=λ/2.16 and e2=λ/5.04 at the 1st frequency, and to e1=λ/3.77 ande2=λ/8.81 at the 2nd frequency.
 4. Transducer according to claim 4,characterized in that the active plate (201) has a thickness such thatit resonates in λ/2 mode at a frequency of 250 kHz and in that the twofrequencies of emission for which the transducer is matched aresubstantially equal to 350 kHz and 150 kHz.